1. Field of the Invention
The present invention relates to an apparatus for ultrasound screening.
2. Description of the Related Art
For a variety of processes, especially those involving the loading, use, or production of bulk materials, especially powders, it is customary to screen the bulk materials produced or used. In this context, it has been known for many years that ultrasound excitation of the screen fabric can substantially enhance the throughput rate. The throughput rate during ultrasound screening depends on the tendency of the screen fabric to become clogged. By the use of ultrasound, the fabric openings are kept free, since the static friction is transformed by the ultrasound movement into the weaker sliding friction and powder bridges are broken up.
According to the prior art, however, the use of ultrasound for ultrasound screening entails a number of conditions. In order to ensure a satisfactory channeling of the ultrasound vibrations into the screen fabric, metallic screen fabrics must be used, and moreover they must precisely fulfill certain fabric tension conditions. In practice, only screen fabrics with mesh below 300 μm can be used at present.
The suitable bulk materials also place limits on the use of known ultrasound screens or limit their efficiency. Moist or wet bulk materials result in heavy attenuation and thus loss of ultrasound action. With other bulk materials there can be an electrostatic build-up, which hinders the throughput rate.
For many years there has been a quest to find ways of introducing the ultrasound into the screen fabric in ever more efficient manner in order to boost the throughput rate which can be achieved with ultrasound screening. Thus, for example, it is known from U.S. Pat. No. 5,386,169 how to undertake an ultrasound excitation on the screen frame, which is then transmitted to the screen fabric stretched in the screen frame. But this method is only practicable for relatively small screens, because with increasing distance of a region of the screen fabric from the screen frame attenuation effects weaken the amplitude of the ultrasound vibration more and more.
Therefore, a switch has taken place, especially for large ultrasound screens, no longer to carry out the ultrasound excitation of the screen fabric through the screen frames, but instead through sound conductors or resonators, i.e., sound conductors tuned to a particular ultrasound frequency, which are arranged on the screen fabric, especially those glued in place. Such screening systems are known, for example, from FR 2 682 050 or DE 10 2006 047 592.
The most varied approaches have been chosen in the effort to ensure a sufficient sonic input on the entire screen fabric, e.g., a consistent exciting of the sound conductor into resonance (see, e.g., DE 44 18 175 A1) or frequency variation about a working point at which the entire system takes up high power from the generator driving the ultrasound converter (see, e.g., EP 2 049 274 B1).
However, it has been shown that these methods also continue to have drawbacks. On the one hand, there is the expense of attaching the sound conductors or resonators and problems in connecting the sound conductors to the screen frame, which is supposed to prevent an unwanted draining of ultrasound energy into the screen frame, and on the other hand the sound conductor must be mechanically supported, especially in the case of screen systems where the screening process is further sustained by an external movement, such as tumble screening.
Finally, there continue to be urgent problems in providing the necessary ultrasound intensity at all places of the screen fabric. These problems specifically manifest themselves in that sticking grains which occur cannot be removed by the ultrasound excitation at all places of the screen fabric. In the case of sound conductors firmly attached to the screen fabric, the energy density and the achieved amplitude of vibration is often not enough to remove sticking grains from the mesh openings.
The problem which the invention solves is to provide a method for ultrasound screening and an ultrasound screen which ensure an improved distribution of the ultrasound excitation over the screen fabric and thus accomplish an improved throughput rate of the screened material.
Accordingly, there is a need for an improved apparatus and method for ultrasound screening.